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<B><A HREF="INTRO.html">INTRO(2)</A></B>		  FreeBSD System Calls Manual		      <B><A HREF="INTRO.html">INTRO(2)</A></B>


</PRE>
<H2>NAME</H2><PRE>
     <B>intro</B> - introduction to system calls and error numbers


</PRE>
<H2>SYNOPSIS</H2><PRE>
     <B>#include</B> <B>&lt;errno.h&gt;</B>


</PRE>
<H2>DESCRIPTION</H2><PRE>
     This section provides an overview of the system calls, their error re-
     turns, and other common definitions and concepts.


</PRE>
<H2>RETURN VALUES</H2><PRE>
     Nearly all of the system calls provide an error number referenced via the
     external identifier errno. This identifier is defined in &lt;<I>sys/errno.h</I>&gt; as

	   extern int * __error();
	   #define errno (* __error())

     The <B>__</B><I>error()</I> function returns a pointer to a field in the thread specif-
     ic structure for threads other than the initial thread. For the initial
     thread and non-threaded processes, <B>__</B><I>error()</I> returns a pointer to a glob-
     al <I>errno</I> variable that is compatible with the previous definition.

     When a system call detects an error, it returns an integer value indicat-
     ing failure (usually -1) and sets the variable <I>errno</I> accordingly.	&lt;This
     allows interpretation of the failure on receiving a -1 and to take action
     accordingly.&gt; Successful calls never set <I>errno</I>; once set, it remains un-
     til another error occurs.	It should only be examined after an error.
     Note that a number of system calls overload the meanings of these error
     numbers, and that the meanings must be interpreted according to the type
     and circumstances of the call.

     The following is a complete list of the errors and their names as given
     in &lt;<I>sys/errno.h</I>&gt;.

     0 <I>Error</I> <I>0</I>. Not used.

     1 EPERM <I>Operation</I> <I>not</I> <I>permitted</I>. An attempt was made to perform an opera-
	     tion limited to processes with appropriate privileges or to the
	     owner of a file or other resources.

     2 ENOENT <I>No</I> <I>such</I> <I>file</I> <I>or</I> <I>directory</I>. A component of a specified pathname
	     did not exist, or the pathname was an empty string.

     3 ESRCH <I>No</I> <I>such</I> <I>process</I>. No process could be found corresponding to that
	     specified by the given process ID.

     4 EINTR <I>Interrupted</I> <I>function</I> <I>call</I>. An asynchronous signal (such as SIGINT
	     or SIGQUIT) was caught by the process during the execution of an
	     interruptible function. If the signal handler performs a normal
	     return, the interrupted function call will seem to have returned
	     the error condition.

     5 EIO <I>Input/output</I> <I>error</I>. Some physical input or output error occurred.
	     This error will not be reported until a subsequent operation on
	     the same file descriptor and may be lost (over written) by any
	     subsequent errors.

     6 ENXIO <I>No</I> <I>such</I> <I>device</I> <I>or</I> <I>address</I>. Input or output on a special file re-
	     ferred to a device that did not exist, or made a request beyond
	     the limits of the device.	This error may also occur when, for
	     example, a tape drive is not online or no disk pack is loaded on

	     a drive.

     7 E2BIG <I>Arg</I> <I>list</I> <I>too</I> <I>long</I>. The number of bytes used for the argument and
	     environment list of the new process exceeded the current limit of
	     65536 bytes (NCARGS in &lt;<I>sys/param.h</I>&gt;).

     8 ENOEXEC <I>Exec</I> <I>format</I> <I>error</I>. A request was made to execute a file that,
	     although it has the appropriate permissions, was not in the for-
	     mat required for an executable file.

     9 EBADF <I>Bad</I> <I>file</I> <I>descriptor</I>. A file descriptor argument was out of range,
	     referred to no open file, or a read (write) request was made to a
	     file that was only open for writing (reading).

     10 ECHILD <I>No</I> <I>child</I> <I>processes</I>. A <B><A HREF="wait.html">wait(2)</A></B> or <B><A HREF="waitpid.html">waitpid(2)</A></B> function was exe-
	     cuted by a process that had no existing or unwaited-for child
	     processes.

     11 EDEADLK <I>Resource</I> <I>deadlock</I> <I>avoided</I>. An attempt was made to lock a sys-
	     tem resource that would have resulted in a deadlock situation.

     12 ENOMEM <I>Cannot</I> <I>allocate</I> <I>memory</I>. The new process image required more
	     memory than was allowed by the hardware or by system-imposed mem-
	     ory management constraints.  A lack of swap space is normally
	     temporary; however, a lack of core is not.  Soft limits may be
	     increased to their corresponding hard limits.

     13 EACCES <I>Permission</I> <I>denied</I>. An attempt was made to access a file in a
	     way forbidden by its file access permissions.

     14 EFAULT <I>Bad</I> <I>address</I>. The system detected an invalid address in attempt-
	     ing to use an argument of a call.

     15 ENOTBLK <I>Not</I> <I>a</I> <I>block</I> <I>device</I>. A block device operation was attempted on
	     a non-block device or file.

     16 EBUSY <I>Resource</I> <I>busy</I>. An attempt to use a system resource which was in
	     use at the time in a manner which would have conflicted with the
	     request.

     17 EEXIST <I>File</I> <I>exists</I>. An existing file was mentioned in an inappropriate
	     context, for instance, as the new link name in a <B><A HREF="link.html">link(2)</A></B> func-
	     tion.

     18 EXDEV <I>Improper</I> <I>link</I>. A hard link to a file on another file system was
	     attempted.

     19 ENODEV <I>Operation</I> <I>not</I> <I>supported</I> <I>by</I> <I>device</I>. An attempt was made to apply
	     an inappropriate function to a device, for example, trying to
	     read a write-only device such as a printer.

     20 ENOTDIR <I>Not</I> <I>a</I> <I>directory</I>. A component of the specified pathname exist-
	     ed, but it was not a directory, when a directory was expected.

     21 EISDIR <I>Is</I> <I>a</I> <I>directory</I>. An attempt was made to open a directory with
	     write mode specified.

     22 EINVAL <I>Invalid</I> <I>argument</I>. Some invalid argument was supplied. (For ex-
	     ample, specifying an undefined signal to a <B><A HREF="signal.html">signal(3)</A></B> or <B><A HREF="kill.html">kill(2)</A></B>
	     function).

     23 ENFILE <I>Too</I> <I>many</I> <I>open</I> <I>files</I> <I>in</I> <I>system</I>. Maximum number of file descrip-
	     tors allowable on the system has been reached and a requests for
	     an open cannot be satisfied until at least one has been closed.

     24 EMFILE <I>Too</I> <I>many</I> <I>open</I> <I>files</I>. &lt;As released, the limit on the number of
	     open files per process is 64.&gt; <B><A HREF="Getdtablesize.html">Getdtablesize(2)</A></B> will obtain the
	     current limit.

     25 ENOTTY <I>Inappropriate</I> <I>ioctl</I> <I>for</I> <I>device</I>. A control function (see
	     <B><A HREF="ioctl.html">ioctl(2)</A></B>)	was attempted for a file or special device for which
	     the operation was inappropriate.

     26 ETXTBSY <I>Text</I> <I>file</I> <I>busy</I>. The new process was a pure procedure (shared
	     text) file which was open for writing by another process, or
	     while the pure procedure file was being executed an <B><A HREF="open.html">open(2)</A></B> call
	     requested write access.

     27 EFBIG <I>File</I> <I>too</I> <I>large</I>. The size of a file exceeded the maximum (about
	     2.1E9 bytes).

     28 ENOSPC <I>Device</I> <I>out</I> <I>of</I> <I>space</I>. A <B><A HREF="write.html">write(2)</A></B> to an ordinary file, the cre-
	     ation of a directory or symbolic link, or the creation of a di-
	     rectory entry failed because no more disk blocks were available
	     on the file system, or the allocation of an inode for a newly
	     created file failed because no more inodes were available on the
	     file system.

     29 ESPIPE <I>Illegal</I> <I>seek</I>. An <B><A HREF="lseek.html">lseek(2)</A></B> function was issued on a socket, pipe
	     or FIFO.

     30 EROFS <I>Read-only</I> <I>file</I> <I>system</I>. An attempt was made to modify a file or
	     directory was made on a file system that was read-only at the
	     time.

     31 EMLINK <I>Too</I> <I>many</I> <I>links</I>. Maximum allowable hard links to a single file
	     has been exceeded (limit of 32767 hard links per file).

     32 EPIPE <I>Broken</I> <I>pipe</I>. A write on a pipe, socket or FIFO for which there
	     is no process to read the data.

     33 EDOM <I>Numerical</I> <I>argument</I> <I>out</I> <I>of</I> <I>domain</I>. A numerical input argument was
	     outside the defined domain of the mathematical function.

     34 ERANGE <I>Numerical</I> <I>result</I> <I>out</I> <I>of</I> <I>range</I>. A numerical result of the func-
	     tion was too large to fit in the available space (perhaps exceed-
	     ed precision).

     35 EAGAIN <I>Resource</I> <I>temporarily</I> <I>unavailable</I>. This is a temporary condition
	     and later calls to the same routine may complete normally.

     36 EINPROGRESS <I>Operation</I> <I>now</I> <I>in</I> <I>progress</I>. An operation that takes a long
	     time to complete (such as a <B><A HREF="connect.html">connect(2)</A></B>)  was attempted on a non-
	     blocking object (see <B><A HREF="fcntl.html">fcntl(2)</A></B>).

     37 EALREADY <I>Operation</I> <I>already</I> <I>in</I> <I>progress</I>. An operation was attempted on
	     a non-blocking object that already had an operation in progress.

     38 ENOTSOCK <I>Socket</I> <I>operation</I> <I>on</I> <I>non-socket</I>. Self-explanatory.

     39 EDESTADDRREQ <I>Destination</I> <I>address</I> <I>required</I>. A required address was
	     omitted from an operation on a socket.

     40 EMSGSIZE <I>Message</I> <I>too</I> <I>long</I>. A message sent on a socket was larger than
	     the internal message buffer or some other network limit.

     41 EPROTOTYPE <I>Protocol</I> <I>wrong</I> <I>type</I> <I>for</I> <I>socket</I>. A protocol was specified
	     that does not support the semantics of the socket type requested.
	     For example, you cannot use the ARPA Internet UDP protocol with
	     type SOCK_STREAM.

     42 ENOPROTOOPT <I>Protocol</I> <I>not</I> <I>available</I>. A bad option or level was speci-

	     fied in a <B><A HREF="getsockopt.html">getsockopt(2)</A></B> or <B><A HREF="getsockopt.html">setsockopt(2)</A></B> call.

     43 EPROTONOSUPPORT <I>Protocol</I> <I>not</I> <I>supported</I>. The protocol has not been con-
	     figured into the system or no implementation for it exists.

     44 ESOCKTNOSUPPORT <I>Socket</I> <I>type</I> <I>not</I> <I>supported</I>. The support for the socket
	     type has not been configured into the system or no implementation
	     for it exists.

     45 EOPNOTSUPP <I>Operation</I> <I>not</I> <I>supported</I>. The attempted operation is not
	     supported for the type of object referenced.  Usually this occurs
	     when a file descriptor refers to a file or socket that cannot
	     support this operation, for example, trying to <I>accept</I> a connec-
	     tion on a datagram socket.

     46 EPFNOSUPPORT <I>Protocol</I> <I>family</I> <I>not</I> <I>supported</I>. The protocol family has
	     not been configured into the system or no implementation for it
	     exists.

     47 EAFNOSUPPORT <I>Address</I> <I>family</I> <I>not</I> <I>supported</I> <I>by</I> <I>protocol</I> <I>family</I>. An ad-
	     dress incompatible with the requested protocol was used.  For ex-
	     ample, you shouldn't necessarily expect to be able to use NS ad-
	     dresses with ARPA Internet protocols.

     48 EADDRINUSE <I>Address</I> <I>already</I> <I>in</I> <I>use</I>. Only one usage of each address is
	     normally permitted.

     49 EADDRNOTAVAIL <I>Cannot</I> <I>assign</I> <I>requested</I> <I>address</I>. Normally results from
	     an attempt to create a socket with an address not on this ma-
	     chine.

     50 ENETDOWN <I>Network</I> <I>is</I> <I>down</I>. A socket operation encountered a dead net-
	     work.

     51 ENETUNREACH <I>Network</I> <I>is</I> <I>unreachable</I>. A socket operation was attempted
	     to an unreachable network.

     52 ENETRESET <I>Network</I> <I>dropped</I> <I>connection</I> <I>on</I> <I>reset</I>. The host you were con-
	     nected to crashed and rebooted.

     53 ECONNABORTED <I>Software</I> <I>caused</I> <I>connection</I> <I>abort</I>. A connection abort was
	     caused internal to your host machine.

     54 ECONNRESET <I>Connection</I> <I>reset</I> <I>by</I> <I>peer</I>. A connection was forcibly closed
	     by a peer.  This normally results from a loss of the connection
	     on the remote socket due to a timeout or a reboot.

     55 ENOBUFS <I>No</I> <I>buffer</I> <I>space</I> <I>available</I>. An operation on a socket or pipe
	     was not performed because the system lacked sufficient buffer
	     space or because a queue was full.

     56 EISCONN <I>Socket</I> <I>is</I> <I>already</I> <I>connected</I>. A <B><A HREF="connect.html">connect(2)</A></B> request was made on
	     an already connected socket; or, a <B><A HREF="send.html">sendto(2)</A></B> or <B><A HREF="send.html">sendmsg(2)</A></B> re-
	     quest on a connected socket specified a destination when already
	     connected.

     57 ENOTCONN <I>Socket</I> <I>is</I> <I>not</I> <I>connected</I>. An request to send or receive data
	     was disallowed because the socket was not connected and (when
	     sending on a datagram socket) no address was supplied.

     58 ESHUTDOWN <I>Cannot</I> <I>send</I> <I>after</I> <I>socket</I> <I>shutdown</I>. A request to send data
	     was disallowed because the socket had already been shut down with
	     a previous <B><A HREF="shutdown.html">shutdown(2)</A></B> call.

     60 ETIMEDOUT <I>Operation</I> <I>timed</I> <I>out</I>. A <B><A HREF="connect.html">connect(2)</A></B> or <B><A HREF="send.html">send(2)</A></B> request failed
	     because the connected party did not properly respond after a pe-
	     riod of time.  (The timeout period is dependent on the communica-
	     tion protocol.)

     61 ECONNREFUSED <I>Connection</I> <I>refused</I>. No connection could be made because
	     the target machine actively refused it.  This usually results
	     from trying to connect to a service that is inactive on the for-
	     eign host.

     62 ELOOP <I>Too</I> <I>many</I> <I>levels</I> <I>of</I> <I>symbolic</I> <I>links</I>. A path name lookup involved
	     more than 8 symbolic links.

     63 ENAMETOOLONG <I>File</I> <I>name</I> <I>too</I> <I>long</I>. A component of a path name exceeded
	     255 (MAXNAMELEN) characters, or an entire path name exceeded 1023
	     (MAXPATHLEN-1) characters.

     64 EHOSTDOWN <I>Host</I> <I>is</I> <I>down</I>. A socket operation failed because the destina-
	     tion host was down.

     65 EHOSTUNREACH <I>No</I> <I>route</I> <I>to</I> <I>host</I>. A socket operation was attempted to an
	     unreachable host.

     66 ENOTEMPTY <I>Directory</I> <I>not</I> <I>empty</I>. A directory with entries other than `.'
	     and `..' was supplied to a remove directory or rename call.

     67 EPROCLIM <I>Too</I> <I>many</I> <I>processes</I>.

     68 EUSERS <I>Too</I> <I>many</I> <I>users</I>. The quota system ran out of table entries.

     69 EDQUOT <I>Disc</I> <I>quota</I> <I>exceeded</I>. A <B><A HREF="write.html">write(2)</A></B> to an ordinary file, the cre-
	     ation of a directory or symbolic link, or the creation of a di-
	     rectory entry failed because the user's quota of disk blocks was
	     exhausted, or the allocation of an inode for a newly created file
	     failed because the user's quota of inodes was exhausted.

     70 ESTALE <I>Stale</I> <I>NFS</I> <I>file</I> <I>handle</I>. An attempt was made to access an open
	     file (on an NFS filesystem) which is now unavailable as refer-
	     enced by the file descriptor.  This may indicate the file was
	     deleted on the NFS server or some other catastrophic event oc-
	     curred.

     72 EBADRPC <I>RPC</I> <I>struct</I> <I>is</I> <I>bad</I>. Exchange of RPC information was unsuccess-
	     ful.

     73 ERPCMISMATCH <I>RPC</I> <I>version</I> <I>wrong</I>. The version of RPC on the remote peer
	     is not compatible with the local version.

     74 EPROGUNAVAIL <I>RPC</I> <I>prog.</I> <I>not</I> <I>avail</I>. The requested program is not regis-
	     tered on the remote host.

     75 EPROGMISMATCH <I>Program</I> <I>version</I> <I>wrong</I>. The requested version of the pro-
	     gram is not available on the remote host (RPC).

     76 EPROCUNAVAIL <I>Bad</I> <I>procedure</I> <I>for</I> <I>program</I>. An RPC call was attempted for
	     a procedure which doesn't exist in the remote program.

     77 ENOLCK <I>No</I> <I>locks</I> <I>available</I>. A system-imposed limit on the number of si-
	     multaneous file locks was reached.

     78 ENOSYS <I>Function</I> <I>not</I> <I>implemented</I>. Attempted a system call that is not
	     available on this system.


</PRE>
<H2>DEFINITIONS</H2><PRE>
     Process ID.
	     Each active process in the system is uniquely identified by a
	     non-negative integer called a process ID.	The range of this ID

	     is from 0 to 99999.

     Parent process ID
	     A new process is created by a currently active process; (see
	     <B><A HREF="fork.html">fork(2)</A></B>).	The parent process ID of a process is initially the
	     process ID of its creator.  If the creating process exits, the
	     parent process ID of each child is set to the ID of a system pro-
	     cess, <B><A HREF="init.html">init(8)</A></B>.

     Process Group
	     Each active process is a member of a process group that is iden-
	     tified by a non-negative integer called the process group ID.
	     This is the process ID of the group leader.  This grouping per-
	     mits the signaling of related processes (see <B><A HREF="termios.html">termios(4)</A></B>)  and the
	     job control mechanisms of <B><A HREF="csh.html">csh(1)</A></B>.

     Session
	     A session is a set of one or more process groups.	A session is
	     created by a successful call to <B><A HREF="setsid.html">setsid(2)</A></B>,  which causes the
	     caller to become the only member of the only process group in the
	     new session.

     Session leader
	     A process that has created a new session by a successful call to
	     <B><A HREF="setsid.html">setsid(2)</A></B>,  is known as a session leader.	Only a session leader
	     may acquire a terminal as its controlling terminal (see
	     <B><A HREF="termios.html">termios(4)</A></B>).

     Controlling process
	     A session leader with a controlling terminal is a controlling
	     process.

     Controlling terminal
	     A terminal that is associated with a session is known as the con-
	     trolling terminal for that session and its members.

     Terminal Process Group ID
	     A terminal may be acquired by a session leader as its controlling
	     terminal.	Once a terminal is associated with a session, any of
	     the process groups within the session may be placed into the
	     foreground by setting the terminal process group ID to the ID of
	     the process group.  This facility is used to arbitrate between
	     multiple jobs contending for the same terminal; (see <B><A HREF="csh.html">csh(1)</A></B> and
	     <B><A HREF="tty.html">tty(4)</A></B>).

     Orphaned Process Group
	     A process group is considered to be <I>orphaned</I> if it is not under
	     the control of a job control shell.  More precisely, a process
	     group is orphaned when none of its members has a parent process
	     that is in the same session as the group, but is in a different
	     process group.  Note that when a process exits, the parent pro-
	     cess for its children is changed to be <B><A HREF="init.html">init(8)</A></B>,  which is in a
	     separate session.	Not all members of an orphaned process group
	     are necessarily orphaned processes (those whose creating process
	     has exited).  The process group of a session leader is orphaned
	     by definition.

     Real User ID and Real Group ID
	     Each user on the system is identified by a positive integer
	     termed the real user ID.

	     Each user is also a member of one or more groups.	One of these
	     groups is distinguished from others and used in implementing ac-
	     counting facilities.  The positive integer corresponding to this
	     distinguished group is termed the real group ID.

	     All processes have a real user ID and real group ID.  These are
	     initialized from the equivalent attributes of the process that
	     created it.

     Effective User Id, Effective Group Id, and Group Access List
	     Access to system resources is governed by two values: the effec-
	     tive user ID, and the group access list.  The first member of the
	     group access list is also known as the effective group ID.  (In
	     POSIX.1, the group access list is known as the set of supplemen-
	     tary group IDs, and it is unspecified whether the effective group
	     ID is a member of the list.)

	     The effective user ID and effective group ID are initially the
	     process's real user ID and real group ID respectively.  Either
	     may be modified through execution of a set-user-ID or set-group-
	     ID file (possibly by one its ancestors) (see <B><A HREF="execve.html">execve(2)</A></B>).  By con-
	     vention, the effective group ID (the first member of the group
	     access list) is duplicated, so that the execution of a set-group-
	     ID program does not result in the loss of the original (real)
	     group ID.

	     The group access list is a set of group IDs used only in deter-
	     mining resource accessibility.  Access checks are performed as
	     described below in ``File Access Permissions''.

     Saved Set User ID and Saved Set Group ID
	     When a process executes a new file, the effective user ID is set
	     to the owner of the file if the file is set-user-ID, and the ef-
	     fective group ID (first element of the group access list) is set
	     to the group of the file if the file is set-group-ID.  The effec-
	     tive user ID of the process is then recorded as the saved set-us-
	     er-ID, and the effective group ID of the process is recorded as
	     the saved set-group-ID.  These values may be used to regain those
	     values as the effective user or group ID after reverting to the
	     real ID (see <B><A HREF="setuid.html">setuid(2)</A></B>).  (In POSIX.1, the saved set-user-ID and
	     saved set-group-ID are optional, and are used in setuid and set-
	     gid, but this does not work as desired for the super-user.)

     Super-user
	     A process is recognized as a <I>super-user</I> process and is granted
	     special privileges if its effective user ID is 0.

     Special Processes
	     The processes with process IDs of 0, 1, and 2 are special.  Pro-
	     cess 0 is the scheduler.  Process 1 is the initialization process
	     <B><A HREF="init.html">init(8)</A></B>,  and is the ancestor of every other process in the sys-
	     tem.  It is used to control the process structure.  Process 2 is
	     the paging daemon.

     Descriptor
	     An integer assigned by the system when a file is referenced by
	     <B><A HREF="open.html">open(2)</A></B> or <B><A HREF="dup.html">dup(2)</A></B>,  or when a socket is created by <B><A HREF="pipe.html">pipe(2)</A></B>,
	     <B><A HREF="socket.html">socket(2)</A></B> or <B><A HREF="socketpair.html">socketpair(2)</A></B>,  which uniquely identifies an access
	     path to that file or socket from a given process or any of its
	     children.

     File Name
	     Names consisting of up to 255 (MAXNAMELEN) characters may be used
	     to name an ordinary file, special file, or directory.

	     These characters may be selected from the set of all ASCII char-
	     acter excluding 0 (NUL) and the ASCII code for `/' (slash).

	     Note that it is generally unwise to use `*', `?', `[' or `]' as
	     part of file names because of the special meaning attached to

	     these characters by the shell.

     Path Name
	     A path name is a NUL-terminated character string starting with an
	     optional slash `/', followed by zero or more directory names sep-
	     arated by slashes, optionally followed by a file name.  The total
	     length of a path name must be less than 1024 (MAXPATHLEN) charac-
	     ters.

	     If a path name begins with a slash, the path search begins at the
	     <I>root</I> directory.  Otherwise, the search begins from the current
	     working directory.  A slash by itself names the root directory.
	     An empty pathname refers to the current directory.

     Directory
	     A directory is a special type of file that contains entries that
	     are references to other files.  Directory entries are called
	     links.  By convention, a directory contains at least two links,
	     `.' and `..', referred to as <I>dot</I> and <I>dot-dot</I> respectively.  Dot
	     refers to the directory itself and dot-dot refers to its parent
	     directory.

     Root Directory and Current Working Directory
	     Each process has associated with it a concept of a root directory
	     and a current working directory for the purpose of resolving path
	     name searches.  A process's root directory need not be the root
	     directory of the root file system.

     File Access Permissions
	     Every file in the file system has a set of access permissions.
	     These permissions are used in determining whether a process may
	     perform a requested operation on the file (such as opening a file
	     for writing).  Access permissions are established at the time a
	     file is created.  They may be changed at some later time through
	     the <B><A HREF="chmod.html">chmod(2)</A></B> call.

	     File access is broken down according to whether a file may be:
	     read, written, or executed.  Directory files use the execute per-
	     mission to control if the directory may be searched.

	     File access permissions are interpreted by the system as they ap-
	     ply to three different classes of users: the owner of the file,
	     those users in the file's group, anyone else.  Every file has an
	     independent set of access permissions for each of these classes.
	     When an access check is made, the system decides if permission
	     should be granted by checking the access information applicable
	     to the caller.

	     Read, write, and execute/search permissions on a file are granted
	     to a process if:

	     The process's effective user ID is that of the super-user. (Note:
	     even the super-user cannot execute a non-executable file.)

	     The process's effective user ID matches the user ID of the owner
	     of the file and the owner permissions allow the access.

	     The process's effective user ID does not match the user ID of the
	     owner of the file, and either the process's effective group ID
	     matches the group ID of the file, or the group ID of the file is
	     in the process's group access list, and the group permissions al-
	     low the access.

	     Neither the effective user ID nor effective group ID and group
	     access list of the process match the corresponding user ID and
	     group ID of the file, but the permissions for ``other users'' al-
	     low access.

	     Otherwise, permission is denied.

     Sockets and Address Families

	     A socket is an endpoint for communication between processes.
	     Each socket has queues for sending and receiving data.

	     Sockets are typed according to their communications properties.
	     These properties include whether messages sent and received at a
	     socket require the name of the partner, whether communication is
	     reliable, the format used in naming message recipients, etc.

	     Each instance of the system supports some collection of socket
	     types; consult <B><A HREF="socket.html">socket(2)</A></B> for more information about the types
	     available and their properties.

	     Each instance of the system supports some number of sets of com-
	     munications protocols.  Each protocol set supports addresses of a
	     certain format.  An Address Family is the set of addresses for a
	     specific group of protocols.  Each socket has an address chosen
	     from the address family in which the socket was created.


</PRE>
<H2>SEE ALSO</H2><PRE>
     <B><A HREF="intro.html">intro(3)</A></B>,	<B><A HREF="strerror.html">perror(3)</A></B>

4th Berkeley Distribution      February 27, 1995			     9
</PRE>
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